1. Field of the Invention
The present invention relates to an improvement in a lens having Fresnel lens surface(s) and a lighting apparatus using the improved lens.
2. Related Art Statement
Conventionally, many of lighting apparatuses used in various optical application instruments for reading and writing a CD or DVD and the like are structured so as to have a predetermined property to focus light emitted from light source(s) such as LED element(s) or lamp(s) by attaching a lens to a support provided with the light source(s), or by forming a lens integrally with the support.
In order to thin such a lighting apparatus, the light source(s) must be disposed close to the lens, and the lens is required to have a greater power of light focusing and a larger aperture, in other words, a smaller F-number. As the lens, there has been used a transparent member formed into a generally semi-spherical convex lens, or a transparent member having a semispherical convex lens part and a prismatic portion in which a plurality of concentric circle-like prisms are arranged on an outer peripheral section of the convex lens part without increasing the thickness of the lens.
A Fresnel lens surface here means a surface which comprises a convex lens part and a series of coaxial fine prisms formed coaxially to a central axis of the convex lens part on an outer periphery of the convex lens part.
In addition, F-number means an inverse number of relative aperture in an optical system such as a lens. Relative aperture means a ratio between a diameter of lens aperture and a focal length of the lens.
FIG. 8 illustrates a first conventional lighting apparatus. The lighting apparatus has a case 60 made of a resin, an LED 61 which is disposed in the case and which corresponds to a light source, and a semispherical lens 62 supported on the case 60 so as to face the LED. Reference number 63 denotes a bonding wire to connect the LED 61 and an electrode not shown in the figure. In the lighting apparatus, the semispherical lens 62 fulfills a good light-focusing function, but there is a problem that the spherical lens 62 increases in likeness because the semispherical lens has a large lens aperture and a large curvature. Thus, the lighting apparatus cannot be miniaturized, particularly in terms of thickness.
FIGS. 9A and 9B illustrate a second conventional lighting apparatus. The lighting apparatus has a case 70, an LED 71 as a light source disposed in the case and a lens 72 supported on the case 70 so as to face the LED and having a refracting-type Fresnel lens surface. Reference number 73 denotes a bonding wire to connect the LED 71 and an electrode not shown in the figure.
The lens 72 has a Fresnel lens surface in which one convex lens 74 and a prismatic portion 75 having a plurality of ring-shaped prisms coaxial with a central axis or optical axis of the convex lens 74 are provided, an inclined surface of each prismatic portion is a straight line or a curved line copying an original lens surface or making a touch of correction to a shape in order to correct a refracting characteristic partially, apexes of the prisms are aligned to have approximately the same height, respectively and the prismatic portion 75 have a shape of wave rings in a plan view. Therefore, the entire thickness of the convex lens is decreased by removing excess areas of the lens, while maintaining the lens functions of the prismatic portion 75 and a central section of the lens.
The lens 72 having the Fresnel lens surface can be decreased in thickness in case of having the same lens aperture and the same F-number as the semi-spherical lens 62, has as shown in FIG. 8, and the lighting apparatus is decreased in thickness accordingly. However, in FIGS. 9A and 9B, an inclination of the inclined surface of each prism is sharper as going to an outer peripheral section of the lens, and a space between the adjacent prisms is also lesser. Therefore, the manufacturing process of surfaces of the prismatic portion 75, in other words, cutting process of dies for lens is difficult, and there is a problem of limitation in the manufacturing process of the prismatic portion, especially at a position out of a radius of the lens, when the lens has a large aperture. The angle between the light incident and the axis of lens increases at the peripheral parts of the lens. As a result, there is a problem that the light efficiency decreases at the peripheral parts.
FIGS. 10A and 10B illustrate a third conventional lighting apparatus. The lighting apparatus has a case 80, a plurality of LEDs 81 disposed in the case, and a lens array member 82 supported on the case 80 so that each lens 83 of the lens array 82 faces each of the LEDs 81. The lens array member 82 comprises a plurality of semispherical lenses 83 arranged independently and close to each other in a planar state. Each LED 81 is disposed on an optical axis of each semispherical lens 83. Meanwhile, reference number 84 denotes a bonding wire to connect each LED 81 and an electrode not shown in the figure. This conventional lighting apparatus has a high illumination effect with an appropriate light focus. However, it is difficult to this the lens array member 82 and lighting apparatus in this conventional example, similar to the first conventional example.
FIGS. 11A and 11B illustrate a fourth conventional lighting apparatus. The lighting apparatus has substantially the same object as in the third conventional lighting apparatus. The lighting apparatus has a plurality of LEDs 91 disposed in a case 90, and a lens array member 92 supported on the case 90. The lens array member 92 has a plurality of Fresnel lens surfaces 93 disposed so that each Fresnel lens surface 93 faces each of the LEDs 91. The lighting apparatus can be thinned, but there is a limitation to the manufacturing process, similar to the second conventional example.
FIGS. 12A and 12B illustrate a fifth conventional lighting apparatus. In the lighting apparatus, a plurality of LEDs 101 which are three LEDs and each of which emits each color of R, G and B, for example here. The three LEDs 101 are disposed close to one another and located directly below a center part of a lens 100 to obtain any color of light. Here, reference number 102 denotes prismatic portion of the lens 100. The LEDs 101 are embedded in the lens 100. A main problem in a lighting apparatus using the lens 100 is that the plurality of LEDs cannot be disposed on an optical axis of the lens simultaneously. Therefore, the lens having a single optical lens cannot optimize the light emissions from all of the LEDs 101.